﻿A 
  STUDY 
  IN 
  CARCINOLOGY. 
  35 
  

  

  moves 
  the 
  coxa 
  outwards, 
  backwards, 
  and 
  slightly 
  upwards 
  ; 
  the 
  innermost 
  

   and 
  slightly 
  more 
  ventral 
  muscle-cavity 
  encloses 
  the 
  adductor 
  muscles 
  whose 
  

   action 
  is 
  the 
  reverse 
  of 
  the 
  abductors. 
  

  

  On 
  comparing 
  the 
  thoracic 
  skeleton 
  of 
  a 
  crab 
  such 
  as 
  Carcinus 
  mcenas 
  

   with 
  that 
  of 
  a 
  crayfish 
  or 
  a 
  lobster, 
  the 
  differences 
  are 
  considerable. 
  The 
  

   •carapace 
  being 
  removed, 
  the 
  epimera 
  are 
  seen 
  to 
  slope 
  inwards 
  towards 
  the 
  

   median 
  dorsal 
  line 
  like 
  the 
  roof 
  of 
  a 
  house. 
  The 
  sterna 
  instead 
  of 
  being 
  

   narrow 
  and 
  keel-like 
  are 
  widely 
  extended 
  laterally, 
  and 
  form 
  by 
  their 
  union 
  

   the 
  broad 
  ventral 
  plastron 
  across 
  which 
  run 
  a 
  series 
  of 
  transverse 
  grooves 
  

   indicating 
  the 
  sternal 
  apodemes. 
  As 
  a 
  consequence 
  the 
  sockets 
  of 
  the 
  limbs 
  

   are 
  placed 
  not 
  ventrally 
  but 
  on 
  the 
  sides 
  of 
  the 
  body, 
  forming 
  a 
  row 
  which 
  is 
  

   not 
  straight 
  but 
  curved 
  upwards 
  at 
  both 
  ends. 
  In 
  some 
  cases 
  the 
  hinder 
  end 
  

   of 
  the 
  row 
  curves 
  upwards, 
  the 
  fourth 
  pereiopods 
  being 
  dorsal 
  in 
  position. 
  

   Although 
  Dromia 
  is 
  in 
  many 
  respects 
  a 
  primitive 
  crab 
  retaining 
  many 
  

   Macruran 
  features, 
  its 
  endophragmal 
  skeleton 
  is 
  so 
  characteristically 
  crab- 
  

   like 
  that 
  it 
  will 
  serve 
  for 
  comparison 
  and 
  contrast 
  with 
  that 
  of 
  the 
  Crayfish. 
  

   PI. 
  4. 
  fig. 
  12 
  is 
  a 
  drawing 
  of 
  the 
  left 
  side 
  of 
  the 
  thorax 
  of 
  Dromia 
  vulgaris 
  

   after 
  removal 
  of 
  the 
  carapace. 
  The 
  thorax 
  has 
  been 
  slightly 
  tilted 
  over 
  to 
  

   the 
  right 
  to 
  bring 
  it 
  as 
  nearly 
  as 
  possible 
  into 
  comparison 
  with 
  PI. 
  4. 
  fig. 
  11, 
  

   and 
  for 
  simplicity's 
  sake 
  the 
  perspective 
  of 
  the 
  ventral 
  surface 
  of 
  the 
  sternum 
  

   has 
  been 
  omitted. 
  Fig. 
  15 
  (PI. 
  5) 
  is 
  a 
  posterior 
  view 
  of 
  the 
  same 
  preparation 
  

   which 
  illustrates 
  some 
  features 
  not 
  clearly 
  shown 
  in 
  fig. 
  12. 
  

  

  As 
  in 
  Astacus, 
  the 
  strophingia 
  and 
  strophidia, 
  the 
  former 
  situated 
  at 
  the 
  

   upper 
  ends 
  the 
  latter 
  at 
  the 
  lower 
  ends 
  of 
  the 
  arthrophragms, 
  serve 
  as 
  

   convenient 
  landmarks. 
  The 
  most 
  conspicuous 
  feature 
  in 
  the 
  crab 
  is 
  the 
  

   great 
  depth 
  of 
  the 
  arthrophragms, 
  especially 
  those 
  between 
  the 
  muscle- 
  

   cavities 
  of 
  the 
  segments 
  of 
  the 
  second 
  and 
  third 
  and 
  third 
  and 
  fourth 
  

   pereiopods 
  (intersegments 
  xii/xiii 
  and 
  xiii/xiv). 
  It 
  is 
  also 
  characteristic 
  

   that- 
  the 
  three 
  posterior 
  arthrophragms 
  slope 
  forward, 
  the 
  two 
  anterior 
  

   arthrophragms 
  backward, 
  in 
  such 
  wise 
  that 
  the 
  flexor 
  muscle-cavities 
  of 
  

   which 
  they 
  form 
  the 
  walls 
  converge 
  towards 
  a 
  point 
  situated 
  in 
  the 
  middle 
  

   of 
  the 
  thorax. 
  A 
  third 
  characteristic 
  feature 
  is 
  that, 
  owing 
  to 
  the 
  low 
  pitch 
  

   of 
  the 
  gabled 
  roof 
  formed 
  by 
  the 
  epimera 
  (see 
  PI. 
  5. 
  fig. 
  15) 
  the 
  abductor 
  

   muscle-cavities 
  lie 
  above 
  and 
  not, 
  as 
  in 
  the 
  Macrura, 
  in 
  the 
  same 
  horizontal 
  

   plane 
  as 
  the 
  adductor 
  cavities. 
  All 
  these 
  features, 
  which 
  go 
  a 
  long 
  way 
  to 
  

   make 
  up 
  the 
  characteristic 
  cancroid 
  facies, 
  are 
  correlated 
  with 
  the 
  habits 
  of 
  

   crabs. 
  They 
  run 
  much 
  more 
  actively 
  than 
  any 
  Macrura, 
  and 
  the 
  attach- 
  

   ments 
  for 
  the 
  muscles 
  of 
  their 
  pereiopods 
  are 
  proportionally 
  increased. 
  To 
  

   provide 
  these 
  attachments 
  the 
  intersegmental 
  apodemes 
  are 
  verv 
  deep, 
  

   particularly 
  on 
  the 
  sternal 
  surface 
  and 
  on 
  the 
  sides 
  of 
  the 
  body, 
  and 
  the 
  

   arthrophragms, 
  instead 
  of 
  being 
  mere 
  flattened 
  bars 
  as 
  in 
  the 
  Macrura, 
  are 
  

   conspicuous 
  partitions 
  extending 
  deep 
  into 
  the 
  body. 
  But 
  their 
  internal 
  

  

  3* 
  

  

  